The disclosure of Japanese Patent Applications No. 2001-076054 filed on Mar. 16, 2001 and No. 2001-371254 filed on Dec. 6, 2000, each including the specification, drawings and abstract, are incorporated herein by reference in their entireties.
1. Field of Invention
The invention relates to a series spot welding method wherein a pair of spaced electrodes is pressed onto a surface of one of two metal plates superimposed on each other and wherein current is caused to flow between the electrodes so as to weld the two metal plates. The invention also relates to a device that performs the series spot welding method. The invention also relates to electrodes employed in the method or the device.
2. Description of Related Art
Direct spot welding has been widely adopted to weld steel plates to each other. In this welding method, two superimposed steel plates are directly pressed while being sandwiched between upper and lower electrode tips, and spot-like welded regions are obtained with the aid of resistance heat generated in the steel plates by causing current to flow therethrough in the direction of thickness. That is, welded regions called welding nuggets (hereinafter simply referred to as xe2x80x9cnuggetsxe2x80x9d) are formed in a region of contact between the steel plates when current flows therethrough. These nuggets serve to spot-weld the steel plates to each other.
While such direct spot welding has been performed, there has also been performed another method called series spot welding. According to series spot welding, while two steel plates that are superimposed on each other are pressed by a pair of spaced electrode tips in one direction, current is caused to flow between the electrodes so that spot-like welded regions are obtained.
In general, a back electrode is disposed on the back side of a steel plate located on the other side of a steel plate to be pressed by electrode tips, whereby a current path extending through one of the electrode tips, the superimposed steel plates, the back electrode, the superimposed steel plates, and the other electrode tip is constituted. By causing current to flow through the current path, the superimposed steel plates are welded in a spot manner as in the case of direct spot welding.
However, according to series spot welding employing such a back electrode, if a region processed in a preceding step corresponds to a location where the back electrode is disposed, another component member or the like mounted in the preceding step interferes with the back electrode. Therefore, the back electrode cannot be disposed in this region. Thus, it is inevitable to perform series spot welding with no back electrode disposed. However, if series spot welding is performed with no back electrode disposed, a nugget of sufficient size cannot be formed, and a blow hole is created. As a result, a problem of insufficient welding strength is caused.
The invention provides a series spot welding method capable of achieving sufficient welding strength even if there is no back electrode, a series spot welding device, and electrodes employed in the method or the device.
A first aspect of the invention relates to a series spot welding method. A first metal plate to be welded by the method of this aspect has two portions described below, namely, a first portion and second portions. The first portion is in contact with the second metal plate when the first and second metal plates are superimposed on each other, and is not welded to the second metal plate. The second portions are away from the second metal plate when the first and second metal plates are superimposed on each other, and are welded to the second metal plate. This method includes the steps of superimposing the first and second metal plates on each other, pressing electrodes having spherical leading edges onto the second portions in a direction toward the second metal plate so as to deform the second portions into a spherical shape, and causing current to flow between the electrodes with the deformed second portions in contact with the second metal plate so as to weld the first and second metal plates to each other.
According to the first aspect of the invention, the second portions are first deformed into a spherical shape by being pressed by the electrodes, and the second portions thus deformed into the spherical shape can then be brought into spot contact with the second metal plate. Thus, a conduction path narrowed down to a spot by the spherically formed second portions is formed between the first and second metal plates. Therefore, the density of current flowing through the conduction path can be enhanced. By causing current to flow through the conduction path, penetration can be caused between the first and second metal plates. That is, a nugget can be formed between the first and second metal plates.
A second aspect of the invention relates to a series spot welding method. A first metal plate to be welded by the method of this aspect has a first portion that is not welded to a second metal plate and a pair of spherical second portions that protrude toward the second metal plate and that are welded to the second metal plate. This method includes the steps of superimposing the first and second metal plates on each other, pressing a pair of electrodes having spherical leading edges onto the second portions respectively, forming the second portions into a spherical shape identical to that of the leading edge of the electrodes and causing current to flow between the electrodes with the second portions formed into the spherical shape in contact with the second metal plate so as to weld the first and second metal plates to each other.
In this method, it is appropriate that the second portions be formed into a spherical shape by being crushed by the electrodes and that the first and second metal plates be then welded to each other. Alternatively, it is also appropriate that the first metal plate formed into the shape of the leading edges of the electrodes (i.e., into the spherical shape) in advance be superimposed on the second metal plate and be welded thereto.
A third aspect of the invention relates to a series spot welding device. This device has a first electrode having a spherical leading edge, a second electrode having a spherical leading edge, a supporting member that supports the first and second electrodes apart from each other by a predetermined distance in the same direction and parallel to each other, a pressing device that presses the first and second electrodes onto superimposed members, and a controller that controls current flowing between the first and second electrodes through the superimposed members, so as to weld the superimposed members each other.
This device makes it possible to carry out the method according to the first or second aspect of the invention.
The electrodes employed in the series spot welding device according to the third aspect of the invention are also included in the scope of the invention.
A structural body welded by the method according to the first or second aspect of the invention is also included in the scope of the invention.
A structural body welded by the device according to the third aspect of the invention is also included in the scope of the invention.
It is also evidently appropriate that a pair of or more second portions be formed in the first metal plate and be welded to the second metal plate at a pair of or more locations by means of the method according to the first or second aspect of the invention or the device according to the third aspect of the invention.